In the fabrication of microchips for use in the electronics and computer industries, a wafer typically comprises a plurality of individual dies, or chips, arranged in a grid pattern. The portions of the wafer between the individual dies are termed "streets." FIGS. 1A and 1B illustrate an exemplary wafer 10. FIG. 1A is a plan view of a wafer, and FIG. 1B is an enlarged view of section 1B in FIG. 1A. Reference numerals 14 denote individual dies, and reference numerals 12 denote the streets separating the individual dies 14. Streets 12 are areas of the wafer where no componentry has been placed and which define the boundaries of each individual die 14. The integrated circuitry and other componentry appears on only one side, e.g., front side 15, of the wafer. The back side (not shown in FIGS. 1A and 1B) has no circuitry or other componentry. The individual dies 14 comprising a wafer are removed from the wafer by sawing through the wafer along all of the streets, thus physically separating the wafer in both axes into the individual dies.
Semiconductor devices incorporating microstructures, also known as micromechanical devices, with moving parts have been disclosed in the prior art. U.S. Pat. No. 5,345,824, issued Sep. 13, 1994 to Sherman et al., discloses a monolithic accelerometer. U.S. Pat. No. 5,326,726, issued Jul. 5, 1994 to Tsang et al., discloses a method for fabricating a monolithic chip containing integrated circuitry and a suspended microstructure. These patents disclose a suspended microstructure for sensing accelerative forces and integrated circuitry for resolving the signal from the sensor into a useful output. The sensor is a variable capacitance capacitor, the capacitance of which changes responsive to acceleration. One node of the capacitor comprises a polysilicon bridge suspended above the substrate on a series of posts. The polysilicon bridge comprises a suspended longitudinal beam having a plurality of fingers extending transversely therefrom. For each beam finger, there is a corresponding stationary finger positioned parallel and in close proximity thereto. The stationary fingers comprise the other node of the capacitor. The bridge and all of the fingers are electrically conductive. The bridge, including the beam fingers, is charged to a different voltage than the stationary fingers. The polysilicon is resilient such that the bridge, comprising the fingers, sways under accelerative force such that the spacing between the beam fingers and the stationary fingers, and thus the capacitance of the sensor, changes. The capacitance signal from the sensor is fed to the resolving circuitry on the same substrate, which creates an output signal indicative of the magnitude of the accelerative force.
When a monolithic accelerometer chip is fabricated, the integrated circuitry on the chip is coated with passivation to protect it. However, the microstructure cannot the passivated, since it must be able to move freely. Typically, the microstructure is positioned essentially in the center of the microchip. Due to the fact that the microstructure is comprised of extremely small polysilicon elements, and the fact that it is not coated with passivation, the microstructure is extremely fragile. Accordingly, great care must be taken during fabrication, up to and including the final packaging steps, not to damage the microstructure. If a wafer including a set of monolithic accelerometer dies was passed through the standard die separation process, the microstructures would be destroyed. The water jet spray used in the sawing process would destroy the microstructures. If any microstructures survived the water spray during the sawing operation, they would be destroyed during the subsequent spraying and brushing in the cleaning operation. Further, if any microstructures survived those two steps, they would be likely to be destroyed in the pick-and-place station by the vacuum equipped arm which picks up the dies and places them in the grid carrier.
A process for separating circuit dies from a wafer, which may include fragile microstructures, is disclosed in U.S. Pat. No. 5,362,681, issued Nov. 8, 1994 to Roberts, Jr. et al. In the disclosed process, the wafer is placed circuit side down on a wafer mount film having holes that correspond to the locations of the microstructures. The wafer is sawn upside down while it is attached to the wafer mount film. After sawing, the dies are removed from the film one at a time in a process where blunt needles push on the circuit side of the die to lift it from the film, while a vacuum tool pulls on the back side. The individual die is flipped over and is placed, circuit side up, on a second wafer mount film. This process is repeated for each die in the sawn wafer.
The disclosed process is generally satisfactory, but has certain disadvantages. The post-saw part of the process requires a significant amount of custom equipment, and the removal of the dies one at a time from the first wafer mount film can be a lengthy process during which the dies are susceptible to particulate contamination and damage. For example, the dies may be damaged by the blunt needles. A trend in micromechanical devices is toward smaller dies and a larger number of dies on each wafer. As dies become smaller, the margin of error in removing the dies from the first wafer mount film is smaller. Furthermore, more time is required per wafer to transfer the dies one at a time.
Accordingly, there is a need for improved methods for separating microcircuit dies from wafers, which have low probability of damaging the dies, which have low risk of contamination of the dies, and which provide high throughput.